The Mechanism Of Nitrogen Addition Affecting Greenhouse Gas Emissions From Rhizosphere And Non-rhizosphere Soils Of Pinus Tabulaeformis Forest

The acceleration of human production activities release a large amount of nitrogen(N)-containing compounds into the atmosphere,which has caused serious environmental issues of N deposition.Previous studies have proved that increasing N deposition significantly changed the matter cycles of ecosystem,the release rate of soil greenhouse gases,and ultimately affected the global climate conditions.However,it is not very clear how N deposition affects quantity and structure of soil microbial community by changing soil chemical properties and root exudates,which in turn affects the release of greenhouse gases from rhizosphere and non-rhizosphere soils.This thesis takes artificial Pinus tabulaeformis forests on the Loess Plateau of China as the research object,and uses 6-year multi-level N addition experiments(0,3,6,9g N m^-2 y^-1)to simulate the increasing N deposition,through field soil sample collection,laboratory chemistry experiment,climatic chamber culture combined with metabolomics and microbiome analysis and other methods,systematically analyzed the effects of N addition on the chemical composition,respiration rate,exudate characteristics of fine root with different diameters(Very fine root,0-0.5 mm;Intermediate fine root,0.5-1 mm,Largest fine root,1-2mm),soil propertiers,microbial community and greenhouse gas emissions of non-rhizosphere soil and rhizosphere soil with different root diameters.The main results achieved are as follows:(1)With the increase of N addition level,the fine root respiration of P.tabulaeformis increased first and then decreased.The respiration rate had the maximum value in 3 g N m^-2y^-1 treatment at 1°C(1.37?mol CO₂ g^–1 s^–1),had the maximum value in 9 g N m^-2 y^-1treatment at 14°C(3.79?mol CO₂ g^–1 s^–1),had the maximum value in 6 g N m^-2 y^-1 treatment at 18°C(4.38?mol CO₂ g^–1 s^–1).Respiration rate gradually slowed down with fine root diameters,and respiration rate of different diameter roots had similar response to N addition.There were significant differences in the contents of chemical properties among fine root diameter classes(P<0.05).It showed that carbon(C),soluble sugar,starch contents and C/N ratio increased with fine root diameters,while N and P contents decreased with fine root diameters.N addition significantly increased fine root N,phosphorus(P),soluble sugar and starch contents,and had the maximum values in 6 or 9 g N m^-2 y^-1 treatments.After treatment of N addition,the P and soluble sugar contents and C/N ratio of Largest fine root had larger changes than those of Very fine root.N and P contents were the most important nutrients that affect fine respiration rate.(2)There were more than 500 kinds of fine root exudates of P.tabulaeformis,most of which belonged to fatty acids,carbohydrates,alcohols and alkylamines.The relative contents of Vey fine root exudates were higher than that of other fine roots,mainly organic acids,fatty acids and lipids(about 26 kinds of exudates).Largest roots produced some exudates as well,and the relative content of carbohydrates and alcohols were higher than other diameter roots.In general,N addition had a promoting effect on the yield of fine root exudates,and most exudates had the maximum values in 6 or 9 g N m^-2 y^-1 treatments.The different exudates caused by N addition treatments were mainly organic acids,amino acids,carbohydrates and fatty acids,and different exudates were dominant in different treatments.Through partial least square discriminant analysis,it was found that Very fine root exudates had large changes among N treatments,and the Largest fine root exudates changed second.(3)N addition had different effects on the chemical properties of rhizosphere soil and non-rhizosphere soil.Low level N addition promoted while high level N addition inhibited the soil organic carbon(SOC),N,ammonium(NH₄⁺),nitrate(NO₃^-)contents of non-rhizosphere soil,and their maximum values were observed in 0 or 3 g N m^-2 y^-1 treatments.The P content of non-rhizosphere soil was significantly promoted,and it had the maximum value in 9 g N m^-2 y^-1 treatment.In rhizosphere soil,N addition increased the NO₃^-content and decreased the P content,while the contents of SOC,N and NH₄⁺increased first and then decreased with the maximum values in 6 g N m^-2 y^-1 treatment.The property contents of rhizosphere soil of Very fine root were more sensitive to N addition than that of Largest fine root.(4)It showed that copy number of microorganisms in rhizosphere soil was higher than that in non-rhizosphere soil by 56.5%,copy number of bacteria in rhizosphere soil of Very fine root was higher than that in rhizosphere soil of Largest fine root by 58.5%,and copy number of fungi in rhizosphere soil of Largest fine root was higher than that in rhizosphere soil of Very fine root by 41.9%.The fungal diversity of non-rhizosphere soil was higher than that of rhizosphere soil,while the difference of bacterial diversity between the rhizosphere and non-rhizosphere soils was not obvious.In rhizosphere soil,fungal and bacterial diversities increased with fine root diameters.The dominant populations in rhizosphere soil and non-rhizosphere soil were varied.There were many predominant microorganisms in non-rhizosphere soil and rhizosphere soil had few predominant microorganisms.Low level N addition had a promoting effect and high level N addition had an inhibitory effect on the copy number and diversity of bacteria,and the maximum values were obsvered in 3 or 6 g N m^-2y^-1 treatments(copy number,68.70×10⁷ g soil^-1).N addition had an inhibitory effect on fungal copy number,and had a promoting effect on fungal diversity,the rhreshold values were observed in 6 or 9 g N m^-2 y^-1 treatments(copy number,30.69×10⁶ g soil^-1).There were different dominant populations in different N addition treatments.In general,6 or 9 g N m^-2y^-1 treatments had the most dominant populations.Microbial community in rhizosphere soil and non-rhizosphere soil had varied responses to N addition.It showed that microbial quantity and bacterial diversity in rhizosphere soil of Largest fine root and non-rhizosphere soil were more sensitive to N addition than that in rhizosphere soil of Very fine root.In terms of dominant populations,N addition caused the most differential microbes in rhizosphere soil of Very fine root,and the least differential microbes in rhizosphere soil of Largest fine root.During microbial changes in rhizosphere soil,root exudates played more important role than soil chemical properties.Specifically,N addition increased the quantity and diversity of microorganisms by directly reducing the exudate yield of Very fine root and Largest fine root,and by directly increasing the exudate yield of Intermediate fine root.These results were related to the high contents of organic acids,fatty acids,alcohols and alkylamines in Very fine root and Largest fine root,and low contents of exudate in Intermediate fine root.In non-rhizosphere soil,N addition inhibited the quantity and diversity of microorganisms by reducing soil nutrient contents.(5)Soil carbon dioxide(CO₂)emission and methane(CH₄)uptake were significantly promoted by N addition(P<0.05),which had the maximum values in 6 or 9 g N m^-2 y^-1treatments(CO₂,623.15 mg C kg soil^-1;CH₄,1794.49?g C kg soil^-1).Soil nitrous oxide(N₂O)emission was inhibited,and the minimum value was observed in 9 g N m^-2 y^-1 treatment(48.63?g N kg soil^-1).The calculation of global warming potential(GWP)by three greenhouse gases found that N addition had a significant promotion effect on GWP in rhizosphere soil and non-rhizosphere soil.The emissions of CO₂ and N₂O from rhizosphere soils with different fine root diameters had similar response to N addition.However,CH₄uptake in rhizosphere soil was faster than that in non-rhizosphere soil,and CH₄ uptake in rhizosphere soil of Very fine root was faster than that in rhizosphere soil of Largest fine root.Moreover,CH₄ uptake in rhizosphere soil of Very fine root was more sensitive to N addition than that in rhizosphere soil of Largest fine root.N addition had a strong direct promotion effect on greenhouse gas emissions in rhizosphere soil of Very fine root and Intermediate fine root.N addition indirectly promoted the gas emission by directly increasing the nutrient content in rhizosphere soil of Largest fine root.While in non-rhizosphere soil,N addition had a direct promotion effect on soil gas emission in general.Hierarchical trait exist not only in fine root physiological indicators,but also exist in rhizosphere soil,and the characteristics of rhizosphere soil with different fine root diameters had varied responses to N addition.It is suggested that more detailed experimental design is needed in the studies of rhizosphere ecological process to correct previous errors.N deposition has the ability to accelerate soil greenhouse gas emission and warming potential in forest ecosystem,and the changing mechanisms of rhizosphere and non-rhizosphere soils are different.It is recommended to pay more attention to the active biochemical reactions in rhizosphere soil of plants in the future studies.